The present invention relates to an ink ribbon tension device suitable for a color printer for thermally transferring a color image onto a printing paper by using an ink ribbon, and more particularly relates to a tension balancer device for an ink ribbon.
A conventional color printer is shown in FIG. 1. An ink ribbon 1 onto which sublimate dyes such as yellow, magenta and cyan (three primary colors) are applied in a repeated pattern at a constant pitch is used in the color printer.
The ink ribbon 1 is wound between a supply reel 2 and a take-up reel 3 and is interposed between a platen 4 and a thermal head 5. A piece of printing paper (i.e., sensitive paper) 6 is loaded while being wrapped around an outer circumference of the platen 4 by a pair of pinch rollers 7. Then, a dye applied surface 1a of the ink ribbon 1 is brought into intimate contact with the printing paper 6 on the outer circumference of the platen 4 in a direction indicated by allow a by the thermal head 5.
In a first printing process, the platen 4 is drivingly rotated in a direction indicated by an arrow b through a constant pitch, and the printing paper 6 is moved at a constant pitch in the direction b. In this case, the ink ribbon 1 is fed at the same pitch as that of the printing paper 6 in a direction, indicated by an arrow c, by a frictional torque between the printing paper 6 and the ink ribbon 1. In synchronism with this, the take-up reel 3 is drivingly rotated in a direction, indicated by an arrow d, through a torque limiter, thereby taking up the ribbon 1 therearound in the direction d. By the first printing process, the yellow dyes of the ink ribbon are sublimated and at the same time thermally transferred onto the printing paper 6 by the heating action of the thermal head 5, so that a yellow image is printed on the printing paper 6. Subsequently, after the thermal head 5 has been separated away from the paper 6 in the direction a', the platen 4 is drivingly rotated in the reverse direction, indicated by an arrow b', through a constant pitch, so that only the printing paper 6 is once returned in the direction indicated by an arrow b'. Then, the thermal head 5 is again pressed against the paper 6 in the direction a, and the planten 4 is again drivingly rotated in the direction b at the constant pitch to thereby effect a second printing process.
By the second printing process, the magenta dyes of the ink ribbon are sublimated and at the same time thermally transferred onto the printing paper 6 by the heating action of the thermal head 5, so that a magenta image is superimposed on the yellow image on the printing paper 6.
In the same manner, a third printing process is effected. Cyan dyes of the ink ribbon 1 are sublimated and at the same time thermally transferred onto the printing paper 6 by the heating action of the thermal head 5, so that a cyan image is superimposed on the yellow and magenta image on the printing paper 6 and finally a synthetic color image is printed on the printing paper 6 with the yellow, magenta and cyan dyes.
However, in the color printer shown in FIG. 1, there is no tension balancer for balancing a tension in a traverse direction of the ink ribbon 1 fed out in the direction c away from the platen 4. Accordingly, in the case where path lengths between opposite edges in the widthwise direction of the ink ribbon 1 between the platen 4 and the take-up reel 3 are different from each other due to non-uniformity in mechanical part precision and assembling precision, when the ink ribbon 1 is wound in the direction d by the take-up reel 3, a tension (i.e., take-up force), applied to opposite edges in the widthwise direction of the ink ribbon, in the direction c would be unbalanced.
As a result, the ink ribbon 1 is obliquely wound around the take-up reel 3, so that creases would be generated in the ink ribbon 1. Otherwise, an oblique drive torque would be applied to the printing paper 6 by the ink ribbon which is being obliquely wound, so that the printing paper 6 would obliquely travel around the outer circumference of the platen, resulting in generation of deformation or distortion in the thermally transferred color image.
On the other hand, in the color printer shown in FIG. 1, also, there is no tensioning means for imparting a tension in the widthwise direction of the ink ribbon 1 fed out in the direction c away from the platen 4. Accordingly, as shown in FIG. 2, creases 9 along the length of the ink ribbon 1 are likely to be generated when the ink ribbon 1 is wound while imparting a tension in the direction d of the ink ribbon 1 by the take-up reel 3. Incidentally, if the tension applied to the ink ribbon 1 would be too strong or too week, then the creases 9 would be generated. Although a suitable tension exists, since in general the take-up reel 3 is wound at the constant torque and the tension of the ink ribbon 1 would be changed in accordance with a change in winding diameter of the ink ribbon 1, it would be difficult to wind the ink ribbon 1 at a constant tension. The conventional printer also suffers from an adverse affect such that the creases 9 along the length of the ink ribbon 1 would cause linear stains to occur on the printing paper 6.
In other conventional color printers, as shown in FIG. 3, a barrel-shaped roller 8 is interposed between the platen 4 and the take-up reel 3 to thereby effect an automatic centering operation of the ink ribbon 1. Even with this system, it would be impossible to well balance the tension (i.e., take-up force), applied to the opposite edges in the widthwise direction of the ink ribbon 1, in the direction c. Also, since the diameter of the barrel-shaped roller 8 has to be increased to some extent, the system needs a large mounting space, which would lead to the physical enlargement of the system, and an unduly strain would be likely to be generated in the path of the ink ribbon 1.